The present invention relates to headphones, and, especially, the technology feasibly applied to passive headphones and also active headphones equipped with a battery, exhibiting a high sound insulation performance against ambient noises.
The widespread of portable music players has brought rapid expansion of the market of headphones to be connected to the players.
Headphones are roughly classified into an inner-ear type inserted into the auricles of a user and an overhead type with a headband put on the head and soft pads attached to or covering the auricles.
The overhead type is advantageous over the inner-ear type for its much larger size of speakers and speaker housings because the weight of headphones is mainly borne by the head of a user via a headband, thus providing higher sound quality.
With the expansion of the market of headphones, the overhead type headphones, which used to be used mainly in a room, have been used in a variety of circumstances, for example, in a vehicle, such as, a train, a bus, an aircraft, etc., or while waiting for someone outside.
These circumstances give a much high level of ambient noises. The sound quality in a circumstance of a high level of ambient noises is thus the subject to improvements, which leads to the demand for headphones with a higher sound insulation performance.
One technique to enhance the sound quality against ambient noises is a noise-canceling function. And, headphones, so-called noise-canceling headphones having the noise-canceling function have been in the spotlight recently.
The noise-canceling headphones are equipped with: a microphone installed in a housing to receive ambient noise components generated around the headphones; and noise-reducing circuitry to generate signals with the opposite polarity of the ambient noise components, combine the opposite-polarity signals with audio signals, and output the combined signals through speakers. The noise-canceling headphones are thus active headphones that make listeners feel that noises are reduced with “opposite-polarity cancellation effects” to the ambient noises.
Another type of known noise-canceling headphones is equipped with: a microphone provided between an ear of a user and a speaker to directly receive sounds that include ambient noises components and are to be given off to the ear; and feedback noise-reducing circuitry to detect the ambient noises components from the difference between the received sounds and an original signal and output audio signals from which the ambient noises components are subtracted, thus reducing the ambient noises components.
The former using the opposite-polarity cancellation effects is called a feed forward type whereas the latter using the feedback circuitry is called feedback type.
Noise-canceling headphones are generally equipped with a power supply, such as, a battery.
With change in circumstances or conditions for the use of headphones, the use of noise-canceling headphones has been spread, not only for enjoying music through speakers but also for sound insulation, as a sound-insulating apparatus, while a user is sleeping in a vehicle, such as, a plane.
When the noise-canceling headphones are used as a sound-insulating apparatus, the noise-reducing circuitry only functions, while no audio signals of music, etc. is being output, to output audio signals with the opposite polarity of ambient noise components, thus making a user feel that ambient noises are reduced.
Accordingly, when used as a sound-insulating apparatus, the noise-canceling headphones provide a user a comfortable sleeping condition with very low ambient noises even in a circumstance with large ambient noises, which is very useful or helpful for passengers of a vehicle with large engine noises, such as, a jet plane.
The noise-canceling headphones exhibit a high ambient-noise reduction performance when the noise-reducing circuitry is functioning. However, the performance is very poor like the passive type when the noise-reducing circuitry is not functioning. The noise-canceling headphones consume the battery a lot when the noise-reducing circuitry is functioning for long hours for a user to sleep, which requires battery replacement often. Then, the user has to carry new batteries, which is very bothersome when he or she is on a trip.
There is thus a strong demand for the noise-canceling headphones exhibiting an ambient-noise reduction performance even though the noise-reducing circuitry is not functioning.
It is also very useful for the passive type (not the noise-canceling type) if it exhibits such an ambient-noise reduction performance even though there is no output audio signals, which can offer a comfortable sleeping condition in a vehicle, such as, a plane.